The present invention relates to improved methods and devices for sensing temperature changes and changes in the magnitude of any other physical field which can produce a temperature change, including but not limited to infrared radiation, especially radiation of wavelengths longer than about 2.0 micrometers (.mu.m), referred to hereinafter as "thermal infrared radiation".
Prior art methods and devices for measuring temperature include the use of probes made of materials having temperature-dependent photo-luminescence properties. One of the earliest system was described in U.S. Pat. No. 2,551,650 of Urbach, and used a photo-luminescent material the luminescence intensity of which was quenched appreciably with an increase of temperature. Luminescence quenching is usually associated with a decrease of the luminescence decay time of the material following excitation of its luminescence by pulsed or oscillatory light of wavelengths within an electronic absorption band characteristic of the material. Since the measurement of a luminescence decay time is usually more accurate and reliable than the measurement of a liminescence intensity (especially in the absence of intensity referencing), some recent temperature measurement techniques using photo-luminescent probes have used the temperature-dependent luminescence decay time as temperature indicator. These decay time techniques were used in a plurality of fiber optic temperature measuring techniques, including among others those described in U.S. Pat. Nos. 4,223,226 and 4,245,507 and in a publication by J. S. McCormack (Electronics Letters 17, 630 [1981]). These prior art techniques have, however, a serious disadvantage: As temperature increase, the signal strength and, hence, the measurement accuracy, decrease. This limits severely the temperature range of operation of probes which have a temperature coefficient of decay time of the order of one percent or better, so a wide temperature range can be achieved only with probes having a significantly lower temperature coefficient of decay time and, hence, a significantly lower sensitivity and accuracy. Other prior art techniques for the optical measurement of temperature include the measurement of the temperature-dependent changes of the spectral distribution of the luminescence of some phosphors, as described in U.S. Pat. Nos. 3,639,765 and 4,061,578, among others.
None of the above techniques are suitable for measuring small temperature changes of the order of 10.sup.-2 kelvins (k) or smaller, as such measurements would require the capability of measuring minute changes of light intensity with an accuracy better than one part in 10,000.
The sensing of infrared radiation is most commonly carried out by electrical sensors. Two main kinds of sensors are: (a) quantum detectors, and (b) thermal detectors. The quantum detectors operate by converting a number N of infrared photons incident on the active surface of the detector into a number qN of free charges (electrons or "holes"), where q is the quantum efficiency of the detector. By contrast, the thermal detectors are essentially electrical temperature sensors which respond to the temperature rise of the active surface of the detector caused by the absorption of the infrared radiation.
There are also infrared sensors which are entirely optical. They are far less common than the electrical sensors, and are used in specialized applications. They also comprise quantum detectors and thermal detectors. The quantum detectors typically require a two-step excitation process wherein a relatively high energy optical "pump" beam excites the molecules or atoms of the detector to an intermediate excited level. Then the infrared photons to be detected further excite these molecules or atoms to a higher energy level, from which they decay to the ground level by emission of visible radiation. The intensity of the emitted visible radiation is then an indicator of the intensity of the infrared radiation.
The optical thermal detectors include, for example, the rare earth-doped luminescent materials subject of U.S. Pat. Nos. 3,639,765 and 4,061,578. These infrared detectors of low thermal mass which are activated by ultraviolet or short wavelength visible radiation and emit luminescence light, the spectral distribution of which is a sensitive function of temperature. The infrared radiation is detected by the temperature increase caused by the absorbed infrared radiation, and its modulation of the sensor luminescence.